Olefins, particularly C2 and C3 olefins, are desirable as a feed source for making derivative products such as oligomers, e.g., higher olefins, and polymers such as polyethylene and polypropylene. Olefin feed sources have traditionally been produced by cracking petroleum feedstocks.
U.S. Pat. No. 5,090,977 discloses a method of making olefins by steam cracking. The method includes separating the olefin product into methane, hydrogen, ethane, ethylene, propylene and C5+ streams. The disclosed separation preferentially produces propylene, and no propane, butane, butene, or butadiene streams are produced.
Oxygenate feed stocks, however, are becoming an alternative to petroleum feed stocks for making olefins, particularly large quantities of ethylene and propylene for the production of higher olefins and plastic materials. In general, the olefins are formed by contacting the oxygenate components with a molecular sieve catalyst to catalytically convert the oxygenates to olefins. Such processes operate, for example, catalytically according to the overall equation 2CH3OH→C2H4+2H2O.
For example, U.S. Pat. No. 4,499,327, discloses a process for making olefins from methanol using any of a variety of silicoaluminophosphate (SAPO) molecular sieve catalysts. The process is carried out at a temperature between 300° C. and 500° C., a pressure between 0.1 atmosphere to 100 atmospheres, and a weight hourly space velocity (WHSV) of between 0.1 and 40 hr−1. The process is highly selective for making ethylene and propylene.
U.S. Pat. No. 6,121,504 also discloses a method of making olefin product from oxygenate feed using molecular sieve catalysts. Water and other unwanted by-products are removed from the olefin product by contacting with a quench medium. After contacting with the quench medium, a light product fraction is obtained which comprises the desired olefins, but also includes dimethyl ether, methane, CO, CO2, ethane, propane, and other minor components such as water and unreacted oxygenate feedstock.
One of the more particularly undesired by-products is dimethyl ether. The problem of removing dimethyl ether from an olefin product stream has not been satisfactorily solved up to now. Special absorber materials that are supposed to remove the dimethyl ether from the product stream have already been considered. However, it is difficult to find a suitable absorber or adsorber material for this purpose.
It is often necessary to reduce or remove the amount of undesirable hydrocarbon by-products that are present in the olefin composition in order to further process the olefins, particularly the ethylene, propylene and butylene. This is because derivative manufacturing processes may use catalysts that are quite sensitive to the presence of certain hydrocarbons. For example, the undesirable by-product dimethyl ether has been found to act as a poison to certain catalysts that are used to convert ethylene, propylene or butylene to other products.
U.S. Pat. No. 4,474,647 discloses, for example, that dimethyl ether can adversely impact the oligomerization of certain olefins. The patent describes a process for removing dimethyl ether from a C4 and/or C5 olefin stream using distillation. The stream is distilled and separated into an overhead and a bottoms stream. The overhead stream contains dimethyl ether, water, and various hydrocarbons, and the bottoms stream contains purified olefins.
U.S. Pat. No. 5,609,734 discloses a method of removing methanol and dimethyl ether from a mixed hydrocarbon stream. The hydrocarbon stream containing the methanol and dimethyl ether is distilled such that the dimethyl ether and methanol are removed in an overhead stream. Additional methanol is recovered in a side stream in which a methanol permeable membrane is used to obtain the additional separation. Purified hydrocarbon is removed from a bottoms stream.
Removal of dimethyl ether from olefin streams is particularly difficult, since very low levels of dimethyl ether can act as catalyst poisons. This means olefin streams such as C2-C4 olefins should contain little if any dimethyl ether, if further catalytic processing of the olefin stream is desired. Therefore, it is highly desirable to find additional methods of removing dimethyl ether from olefin streams.